(1) Field of the Invention
The present invention relates to image compression encoding and decoding.
(2) Related Art
Advances in network technology and multimedia applications in recent years have created a greater need to handle large amounts of data, such as image files.
Image files generally include a large amount of data, and effectively need to be compressed before transmission. To address this need, image compression methods which use region division (hereinafter xe2x80x9csegmentationxe2x80x9d) have been developed.
Image compression methods that use segmentation use a predetermined judgement standard (called a xe2x80x9cdivision judgement conditionxe2x80x9d) to divide image regions into segments.
As an example method of processing an image region, Image Coding by Adaptive Tree-Structured Segmentation, IEEE, Trans. Info., Vol.38, No.6, pp.1755-1767 (1992) teaches a method for dividing a region into convex polygons, while Image Compression via Improved Quad-tree Decomposition Algorithms, IEEE Image. Proc., Vol.3, No.2, pp.207-215, (1994) teaches a method for quadtree division.
FIG. 1 is a representation of quadtree division.
As shown in FIG. 1, quadtree division uses a predetermined division judgement condition to judge whether a square region should be divided. On judging that division is necessary, this method divides the square region into four square subregions The method then repeats the judgement for the resulting subregions. In this way, the method generates subregions called xe2x80x9csegmentsxe2x80x9d.
In the example shown in FIG. 1, the first division generates four segments. Of these, three segments are subjected to a second division. Of the segments produced by the second division, only segments that conform to the division judgement condition are subjected to a third division.
One conventional example of the division judgement condition uses the distribution of luminance values in the segment. Note that in this specification, xe2x80x9cluminancexe2x80x9d also includes the concept of xe2x80x9cdensityxe2x80x9d. This method may judge whether a difference between the highest and lowest luminance values is within a given threshold, and has an advantage in that it uses little calculation. Another conventional method uses a calculated amount of error (such as the difference of squares) for when a mean value is used in place of every element in the segment. This second method has a different advantage, however, in that users can control the quality of the final reproduced image using the information-to-noise ratio when quadtree division is performed, the compressed image data is composed of segmentation information and luminance information. For the example shown in FIG. 1, the segmentation information is xe2x80x9c1,1110,0011,0010,0110xe2x80x9d, where xe2x80x9c1xe2x80x9d is the division code and xe2x80x9c0xe2x80x9d the no-division code. The luminance information, meanwhile, 1:1 corresponds to each segment when division is complete.
When using the quadtree division method, the greater the extent to which large regions are left undivided, the smaller the data size of the compressed image data, or in other words, the higher the compression rate.
In fields such as medicine, however, images need to be reproduced with a very high resolution to allow accurate representation of detail. As a result, there is a very strong demand in lossless or near-lossless image compression. Lossless compression refers to reversible compression where the pre-compression image will be faithfully reproduced after decoding with no loss of image quality.
One example of a reversible image compression technique that uses segmentation is taught by MDL Genri to 2 Bunkikouzou Segumenteeshon wo Mochiita Gazou no Mubizumi Fugouka Arugorizumu (xe2x80x9cImage Encoding Algorithm that uses MDL Principles and a Dual-Tree Structurexe2x80x9d) published in Shin Gakuron (D-II), Vol.J80-D-II, No.2, pp.415-425, 1997).
When compressing an image using segmentation, there is the problem that improving the detail in the reproduction image inevitably results in division into segments of large areas in the original image where there are only minute differences in luminance values, or in other words, large areas may be regarded having a uniform luminance value. This significantly reduces the compression rate, and is major drawback for reversible (lossless) and quasi-reversible (near-lossless) compression.
The following is a description of the idea of xe2x80x9cprogressivenessxe2x80x9d.
Progressive reproduction refers to a process whereby a low-resolution image is initially displayed at the start of input or decoding of a bitstream, with the details of the image being slowed added as the input or decoding progresses. The reproduction image is finally displayed with the desired resolution.
This kind of reproduction can be highly effective, and is particularly valuable when searching through images, for example.
Compressed image data obtained through an image compression technique using segmentation where each segment has its own luminance value is unsuited to progressive reproduction. When such data is decoded, even if it were possible to produce the reproduction image in accordance with gradual changes in luminance values, it would still be difficult to perform progressive reproduction in accordance with the changes in resolution and in the shapes of segments.
In view of the stated problems, it is necessary to provide a decoding apparatus with a separate component to enable the progressive reproduction of image data that has been compressed in this way. In the third document cited above, progressive reproduction is realized by using a 2-path system where a differential image for differences between the original image and the resemblance image is encoded separately to the resemblance image.
It is an object of the present invention to provide an image compression apparatus that performs reversible compression without being limited to a low compression rate of data as in the conventional art. Such image compression apparatus generates compressed image information that is suited to progressive image reproduction. At the same time, the present invention aims to provide an image decoding apparatus that decodes compressed image data generated by this kind of image compression apparatus.
To achieve the stated object, the image compression apparatus of the present invention performs image compression using a first segmentation method on a first plane set that is composed of at least one consecutive bit plane in block image information in the original image information. By doing so, the image compression apparatus generates first region information composed of first segmentation information showing a segmentation result for a block, and first luminance information showing luminance information for each segment produced by the segmentation. The present image compression apparatus also performs image compression using a second segmentation method on a second plane set that is composed of at least one consecutive bit plane adjacent to the first plane set at a lower bit position in the block image information. By doing so, the image compression apparatus generates second region information composed of second segmentation information showing a segmentation result for the block, and second luminance information showing luminance information for each segment produced by the segmentation. The present image compression apparatus then generates compressed image information based on the first region information and second region information and outputs the compressed image information.
Note that the expression xe2x80x9cbit planexe2x80x9d refers to a level (plane) composed of luminance bit values at a corresponding bit position in the luminance values.
The present image decoding apparatus reads first segmentation information and first luminance information from the inputted compressed image information, uses the first segmentation information to perform segmentation of the block according to a first segmentation method, and assigns the first luminance information to the segments produced by the segmentation to decode the first plane set.
The present image decoding apparatus also reads second segmentation information and second luminance information from the inputted compressed image information, uses the second segmentation information to perform segmentation of the block according to a second segmentation method, and assigns the second luminance information to the segments produced by the segmentation to decode the second plane set.
The image decoding apparatus then uses the decoding results of the first decoding unit and the second decoding unit to generate decoded image information.
With the image compression apparatus and the image decoding apparatus described above, segmentation is separately performed for the first plane set and the second plane set, so that even if there are variations in the luminance bit values in the second plane set but little variation in the luminance bit values in the first plane set, large decreases in the compression rate that conventionally occurred when performing reversible or quasi-reversible compression can be avoided.
The above construction has a particularly large effect when the luminance values in a block of original image data are almost uniform, so that there is little variation in luminance values.
The second plane set is decoded after first decoding the first plane set, so that progressive image reproduction can easily be achieved without requiring the addition of other components.
The result of segmentation of the block for the first plane set may be set as a standard when the image compression apparatus performs image compression according to the second segmentation method to generate the second segmentation information. The result of segmentation of the block for the first plane set may also be set as a standard when the image decoding apparatus performs segmentation of the block using the second segmentation information before assigning the second luminance information to the segments produced by the segmentation to decode the second plane set.
By operating in this way, the image compression apparatus and image decoding apparatus reflect the result of the segmentation of the first plane set in the segmentation performed for the second plane set. This can reduce the data amount of the second segmentation information, and so further increase the compression rate.
The stated first object can also be achieved by an image compression apparatus for producing compressed image information by compressing original image data that includes block image information, the block image information associating each pixel in a block composed of a plurality of pixels with a luminance value expressed using a natural number k of bits, where kxe2x89xa72, the image compression apparatus: performing compression processing on a first plane set using a predetermined segmentation method, the first plane set being composed of at least one consecutive bit plane that includes a highest bit plane in the block image information in inputted original image data, to generate first region information composed of first segmentation information showing a segmentation of the block and first luminance information showing luminance bit values that each correspond to a different segment produced by the segmentation of the block; performing compression processing on an nth plane set using the predetermined segmentation method and a standard that is a result of a segmentation of blocks for an (nxe2x88x921)th plane set in the block image information, the nth plane set being composed of at least one consecutive bit plane that is adjacent to the (nxe2x88x921)th plane at a lower bit position in the block image information, to generate nth region information composed of nth segmentation information showing an nth segmentation of blocks and nth luminance information showing luminance bit values that each correspond to a different segment produced by the nth segmentation of the block, for each value of n from 2 to a predetermined number that is no greater than k; and generating and outputting compressed image information based on the first region information and the nth region information for every value of n from 2 to the predetermined number.
Corresponding to the above, an image decoding apparatus decodes the compressed image information generated by the image compression apparatus, the image decoding apparatus: reading the first segmentation information and the first luminance information from the inputted compressed image information; performing segmentation according to the predetermined segmentation method for blocks using the first segmentation information, and for decoding the first plane set by assigning the first luminance information to segments obtained as a result of the segmentation; repeating a process reading the nth segmentation information and the nth luminance information from the inputted compressed image information, for each value of n from nxe2x88x922 to n=the predetermined number; performing segmentation according to the predetermined segmentation method for blocks using the read nth segmentation information with a result of segmentation for the (nxe2x88x921)th plane set as a standard, and for decoding the nth plane set by assigning the nth luminance information to segments obtained as a result of the segmentation, in doing so decoding every plane set from n=2 to n=the predetermined number; and generating decoded image information based on a decoding result for the first plane set and decoding results for the 2nd to the (predetermined number)th plane jets.
The above image compression apparatus and image decoding apparatus repeatedly perform a process where segmentation is performed for each plane set using the results of segmentation on upper plane sets. This increases the effects of the present invention.